Manufacture of semiconductor devices typically involves a series of processes in which various layers are deposited and patterned on a substrate to form a device of the desired type. Line and space patterns in photoresist are often created to form microelectronic devices. Smaller critical dimensions (CD) for both lines and spaces allow faster circuitry to be created. Deep-ultraviolet (DUV) photolithography is a standard imaging technique for the production of advanced integrated circuits using chemically amplified (CA) resists. Refinements to the physical and chemical properties of the CA resists are important to take advantage of advanced lithography systems, such as extreme ultraviolet (EUV) photolithography.
FIG. 1 is a flowchart illustrating a typical photolithography process. In the photolithography process, a layer of photoresist material is deposited over an underlying layer (formed above a substrate or wafer) that is to be etched. Spin coating is a standard method to apply the photoresist material. The substrate is then subjected to a baking process (also referred to as post-apply bake) to relieve stresses caused by the sheer forces encountered in the spinning process, and remove excess solvent. The photoresist layer is then selectively exposed to radiation (e.g., ultraviolet radiation) with certain regions protected by a mask to form a latent image in the resist layer. The substrate is then subjected to a post-expose bake followed by the development process, in which a photoresist developer, a solution that erodes areas of the photoresist layer exposed to the radiation, is applied to the substrate. The substrate is then rinsed to remove the developer solution, and dried, for example by a spin-dry process.
Image formation in a positive-tone CA photoresist requires conversion of the polymer from an insoluble to a soluble form. Initially, irradiation of the resist triggers a photochemical reaction that produces an acidic product. The proton (i.e., acid catalyst) from this acid binds to an ester functional group attached to the polymer. The protonation induces fragmentation of the ester group that causes the ester to be replaced with a carboxylic acid and an unstable, transient carbocation to be formed. The carbocation releases a proton, which then protonates another ester group to repeat the process, resulting in the deprotection of many ester groups in the resist. When in contact with an aqueous developer, the carboxylic acid groups formed during deprotection will ionize, rendering the polymer soluble.
One of the most difficult challenges for chemically amplified resists is meeting the dense line/space resolution targets. For current commercially available high activation energy resists, high bake temperatures are required for deprotection of the resist. Moreover, the acid catalyst will have large diffusion lengths at high temperatures, which limits the resolution of the resist. Alternatively, low activation energy resists are available, which undergo deprotection at lower temperatures relative to high activation energy resists. The diffusion of the acid catalyst may be lower in low activation energy resists, thereby resulting in higher resolution. For example, hydrolysis of acetal protecting groups is a low activation energy reaction and therefore will occur at low temperatures. Acetal/ketal groups will undergo a hydrolysis reaction in the presence of an acid as shown in FIG. 2. However, one problem with typical low activation energy resists is that because the deprotection reaction does not require high temperatures, outgassing of the deprotection groups can occur at temperatures below that of the post-expose bake. For example, outgassing can occur during the radiation exposure process.